This invention relates in general to polyimide resins and, more specifically, to modified polyimide resins suitable for use as high performance adhesives.
Prior U.S. Pat. Nos. 4,161,477, 4,183,838 and 4,183,839 (to John Gagliani, one of the coinventors of this application) disclosed and claimed certain polyimide compositions which are flame resistant and useful as coatings and adhesives.
The coating and adhesive compositions described in the above-mentioned prior patents are made by first preparing a suitable bisimide by reacting an aromatic tetracarboxylic acid dianhydride with a cyclic amide or oxime. The ratio of oxime to dianhydride is preferably in the 2.3:1 ato 2.7:1 range and the imidization reaction is preferably conducted at a temperature of 170.degree.-200.degree. C. for 20-60 minutes.
The polyimide forming material is then prepared by dissolving the bisimide in an inert solvent; then adding thereto a suitable diamine, producing a viscous fluid containing an intimate, unpolymerized mixture of N-substituted cyclic bisimide dicarboxylic acid and diamine which is capable of being converted to a high molecular weight polymer through an exchange reaction by the application of heat.
The solution is coated onto a surface and polymerized by heating to a temperature in the 177.degree.-316.degree. C. range for 30 minutes to 5 hours. In order to keep the cure period reasonably short, it was necessary to perform a final cure for at least about 30 min. at about 316.degree. C. The following is exemplary of the exchange reaction which occurs: ##STR1## where n is a positive integer.
The resulting coating is tough, highly adherent to various surfaces, with very few pinholes or bubbles. It has excellent peel strength and is resistant to high temperatures, peeling and abrasion.
The prior coating material, however, was not fully satisfactory for many applications. Curing tended to require either excessively high temperatures, in the 300.degree. C. area, or very long curing periods at somewhat lower temperatures. The final adhesive layer tended to be somewhat more rigid and brittle than desired, lacking the ideal adhesive elasticity and flexibility.
Thus, there is a continuing need for improved polyimide adhesives having a better combination of low processing temperatures, durable, flexible bends and resistance to high ambient temperature after curing.